The present invention relates generally to an optical information processor for optically recording and/or reproducing information, and more particularly concerns an information tracking apparatus for tracing information by light beams or spots with high accuracy.
For reading out information from a disc surface in which information is previously recorded in a spiral track pattern as is in the case of an optical video disc, an optical audio disc and the like systems, a tracking function or control is required for causing a writing/reading light spot to trace the information track with accuracy. The tracking control in turn requires an operation for detecting positional deviation of the light spot from the information track and a control operation for displacing the light spot in dependence on a magnitude of the detected deviation and the direction thereof to correctively cancel the deviation.
In the optical video disc systems which are currently commercially available, a tracking control system is practically adopted in which deviation of the light spot from the information track or groove (hereinafter also referred to as the tracking error) is detected by making use of two auxiliary light spots in addition to the main light spot which serves as writing/reading spot. For example, reference is to be made to Japanese Laid-Open patent application No. 44-50954. The principle of this known tracking control system will briefly be reviewed below. Referring to FIG. 1a of the accompanying drawings, an information reading light spot 1 is positioned at a mid point between two auxiliary light spots 2 and 3 with an equal distance l thereto. It will further be noted that the centers 5 and 7 of the auxiliary light spots 2 and 3 are deviated to the right and to the left (as viewed in the drawing), respectively, by a same distance .delta. from a virtual center line 8 which passes through the center 6 of the readout light spot 1 and extends in parallel with the direction in which the information track is moved forwardly as indicated by a blank arrow. Thus, the light spots 2 and 3 may be considered to sandwich the light spot 1 in directions which are both parallel and perpendicular to the information track or groove. Photodetectors for receiving light rays reflected from the disc illuminated by the light spots are disposed on an image plane of an objective lens or lenses for focusing the reflected light rays. More specifically, referring to FIG. 1b, images of the light spots 2, 1 and 3 are focused onto the photodetectors 12, 11 and 13, respectively, through associated objective lenses (not shown). The output signals of the photodetectors 12, 11 and 13 are amplified by associated amplifiers 14, 15 and 16 to constitute output signals A, B and C, respectively. In the case of an optical video disc, the information track denoted by a numeral 4 is constituted by a series of pits (i.e. elongated or circular holes each having a depth corresponding to a quater wavelength of a light source) pressed into the disc surface in a phased array and representing information. Since the information signal recorded on the video disc is derived through frequency modulation of the video signal (whose band covers a range from DC to 4 MHz), it is possible to detect variations or changes in the DC component to be utilized for the tracking control without being subjected to influence of the information signal by selecting the band of the amplifiers 14 and 16 to be sufficiently narrow as compared with that of the information signal.
Assuming now that behavior or variation of the DC component of the output signal B occurring upon traversal of the reading light spot 1 across a track is represented as a function of deviation from the center of the track (i.e. tracking error), the behavior will be such as indicated by a double-dotted broken line curve B in FIG. 2. The output signals A and C correspond to the output signal B shifted to the right (i.e. in the positive direction) and to the left (i.e. in the negative direction), respectively, by the distance .delta. along the abscissa symmetrically to the ordinate, since the auxiliary light spots 2 and 3 corresponding to the output signals A and C are deviated symmetrically from the reading light spot 1, as described above. This means that the rightward deviation or displacement of the track 4 as viewed in FIG. 1a causes the quantity of reflected light of the auxiliary spot 2 to be decreased while the quantity of reflected light of the spot 3 is increased, and vice versa.
For detecting the tracking error, the output signals A and C may be supplied to the inputs of a differential amplifier whose output signal will then be such as indicated by a broken line curve in FIG. 3. The tracking control is made with the aid of light spot control means such as a light beam deflector or the like in such a manner that the output signal of the differential amplifier becomes zero, i.e. the quantities of reflected light of the auxiliary spots 2 and 3 are equal to each other. As will now be appreciated, the tracking control system mentioned above can certainly assure the tracing of the information track by the reading light spot in a stabilized manner, so far as information is previously recorded as in the case of the optical video disc or the optical audio disc. However, in the case of an information processor commonly referred to as the digital optical disc system which is destined to record and reproduce digital information, video information and others on a real-time base, the hitherto known tracking control apparatus suffers a shortcoming that the accurate tracking (i.e. track tracing operation) can not be attained in the recording mode operation. This problem will be scrutinized below.
In a typical one of the known digital optical disc systems, recording of information on a disc composed of a photosensitive information recording medium (e.g. a metallic film) formed on a disc substrate through vapor deposition is accomplished in such a manner that light from a high-power laser is focused in the form of a light spot having a diameter of the order of 1 .mu.m on the surface of the spinning disc with the intensity of the laser beam being modulated in accordance with information to be recorded, as the result of which information is recorded on the recording medium in the form of holes referred to as pits thermally produced in the disc surface in a spiral or concentric circular pattern. In the playback operation, the information carrying medium of the disc is irradiated with a focused light beam of a low-power laser to pick up the recorded information in terms of variations in the quantity of light reflected from the pits. For particulars, reference may be made to an article titled "Ten Billion Bits Fit Onto Two Sides of 12-inch Disc", Electronics, No. 23 ( 1978), p. 75. This information processor system is typically inplemented in such a structure as shown in FIG. 4. Referring to this figure, a disc 103 having a diameter on the order of 30 cm is rotated in the direction indicated by an arrow around a rotating shaft 104 driven by an electric motor 105. An optical head 102 which is constituted by a laser light source, an optical system for guiding laser light from the light source to the disc 103 and photodetectors for detecting light reflected from the disc is mounted on a swing arm actuator 101 to be movable in a radial direction of the disc 103. Recording and playback operations of the optical information processor of the above mentioned structure will be described below by referring to FIG. 5 which shows in a fragmental enlarged view a structure of a disc used to this end.
A groove 113 of a concaved form in section referred to as the guide groove having predetermined width and depth is formed in a spiral or concentric circular pattern in a ultraviolet-hardenable resin layer deposited on a disc substrate 111 of a glass or plastic material. A metallic film 110 is formed over the groove forming resin layer through vapor evaporation. Laser light emitted from the optical head 102 is focused in the form of a spot and moved along the guide groove 113 to thereby record information in the form of pits 112 in the manner described above. In the playback mode, the information carrying disc surface is illuminated with the light spot along the guide groove 113 to detect the quantity of reflected light. Additionally, signals for controlling the main light spot are also derived on the basis of the quantity of reflected light.
The light spot control signals include primarily an off-focus detection signal representative of the out-of-focus state of the light beam due to vertical vibrations of the disc and a tracking error detection signal representative of deviation or error between the center of the light spot and that of the guide groove. These two signals are derived from light rays reflected from the metal film at locations other than the information pits.
Returning to FIG. 1a, the auxiliary spots 2 and 3 as well as the information writing/reading or main spot 1 are formed by splitting a light beam emitted from a single laser light source into the corresponding number of beams by means of a diffraction grating. The ratio of light intensity distribution of the auxiliary spot to that of the main or information writing/reading spot is set at about 1/10 with a view to increasing the light utilization for the main or information writing/reading spot. In the recording mode, the laser power is increased for forming the pits. At this time, the tracking operation for causing the main or writing spot 1 to follow the guide groove 113 has to be carried out simultaneously with the recording operation. However, in the case of the recording mode, the tracking error can not be detected with accuracy by the hitherto known method, giving rise to a problem.
More specifically, it is assumed that the direction indicated by the arrow in FIG. 1a is tangential to the rotating direction of the disc. As the pits are formed by the main or writing light spot 1 having intensity modulated in accordance with information to be recorded in the recording operation, the reproduced signal originating in the auxiliary spot 2 is subjected to the influence of the formed information pit and is decreased in magnitude. As the consequence, the output signal of the photodetector 12 amplified by the amplifier 14 varies in such a manner as represented by a solid line curve A' in FIG. 2. Under the circumstances, the output signal of the differential amplifier which represents the difference between the outputs of the amplifiers 14 and 16 (this difference corresponds to the tracking error signal) is modified as indicated by a solid line curve in FIG. 3 which is remarkably deviated from the broken line curve representative of the desirable tracking error signal. Under the circumstances, the intrinsic center position 0 of the track is shifted to a position indicated by 0' as shown in FIG. 3, involving a corresponding offset in the tracking error signal. Accordingly, when the tracking servo control is performed on the basis of this detected signal (solid line curve), the center of the main or writing light spot is positioned at the offset point 0', as the result of which the information pits are recorded at incorrect positions, to a serious drawback.